A new mechanism has been formulated for the proteolytic enzyme Carboxypeptidase A. It features recognition of the stereochemical consequences of torisional activation of the peptide linkage. Experiments have been designed to confirm this new biochemical principle. A series of substrate analogs has been conceived, the enzymic activity of which has been predicted. The biomedical significance is that a basis is provided for rational design of agents which will modify protein metabolism. Far reaching chemotherapeutic consequences are envisioned in the areas of antibiotic, antiviral and antitumor research.